Because of thermal instability, fuels used for jet propulsion, tend, when heated, to leave a residue that builds up and clogs the orifices, nozzles, and conduits leading to the combustion chamber. Consequently, means have been devised for rating the thermal stability of the jet fuels.
In an earlier method for evaluating the thermal stability of jet fuel, the fuel was passed by a heated metal tube at a constant rate for a specified period of time. To keep the fuel in its liquid state, the fuel was under pressure during the test run. After the test run, the metal tube was removed from the test apparatus and the film deposited on the tube was rated by comparison of the film's color with an ASTM color standard.
In another method for evaluation of the jet fuel's thermal stability, the metal tube, after the test run, is removed from the apparatus and the film deposited on the tube is rated with an instrument known as a "tube deposit rater". To those familiar with the evaluation of jet fuel properties, the tube deposit rater is familiarly referred to as the "TDR". In the tube deposit rater, visible light is directed at the film deposit on the metal tube and the relative intensity of the reflected light is indicated on a calibrated meter. The meter readings range between 0 and 50 and the readings are inversely related to the intensity of the reflected light. The less the intensity, the higher the reading. When rating the thermal stability of a jet fuel, the metal tube, after the test run, is placed in the TDR and the length of the tube is scanned until a position is located at which the maximum meter reading is obtained. The reading at the position is then the rating for the fuel. The basis for that procedure is the assumption that the reflected light intensity decreases monotonically with increasing film thickness so that the rating corresponds to film thickness. The conventional TDR employs a photo detector whose region of sensitivity to the light spectrum extends from 400 to 750 nm (nanometers) with maximum sensitivity at 550 nm. That region coincides with the region where large polycyclic aromatic compounds undergo transitions by the absorption of light. It is believed that those molecules are produced in the thermal oxidation of hydrocarbon fuels and that because of their low solubility the compounds form a deposit on the heater tube. Consequently, it was reasoned that, the thicker the deposit, the more light was absorbed by the large polycyclic aromatic molecules and the less light was reflected so that the intensity of the reflected light decreased in direct relation to the thickness of the deposit. However, evidence has accumulated which indicates that ratings made with the conventional TDR are inaccurate because those ratings do not correlate with data obtained in the testing of jet engines.